In applications such as pipe seals, the governing specifications require the material to have a low compression set. Thermoset rubbers (TSR), those vulcanized or cross-linked after molding, have typically been used. Thermoplastics and thermoplastic elastomers have been proposed to replace TSR in many applications, particularly dynamically vulcanized thermoplastic elastomers, e.g., thermoplastic vulcanizates (TPV). A TPV typically consists of a vulcanized rubber particle phase dispersed within a continuous thermoplastic phase. Those currently proposed to meet typical pipe seal specifications are grades having a hardness of approximately 45-65 Shore A. To form a ring member from an extruded profile, there must necessarily be a joint or weld. The softer grades of TPVs do not always heat weld in a welding process with adequate strength as compared to a vulcanized splice in a TSR. This is often due in soft TPVs to the higher content of dispersed rubber phase and decreased content of hard thermoplastic. In addition, the joint or weld, can provide a leak path both during pressure and vacuum testing of the piping system and its subsequent use if the weld joint is not effective.
Seals for small pipe diameters (e.g., 12 inch diameter) can be prepared by injection molding which avoids a need for a weld. However, seals having large diameters (e.g., greater than 12 inch diameter) or with lower unit volume are not typically suitable for injection molding processes due to high tooling and equipment costs. Instead, extruded seal material profiles are cut to length, formed into a ring, and then welded or joined through a thermal welding process or vulcanized splicing. Thus there exists a need in the art for an improved process to provide larger diameter pipe seals that can be fabricated from thermoplastic materials.
In the thermal welding process excess thermoplastic extends upon joining of the ends thus causing leaking or substandard seal performance. Thus, the heat-welded joint of thermoplastic, particularly thermoplastic elastomer, has substandard performance with respect to the thermoset rubber seals that utilize a vulcanized splice with little excess. Thus TPE seals have been inadequate for more demanding applications such as sewer pipes due to weld quality and long-term performance. The present invention is directed to overcoming these and other problems.